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The Science of Sex: Survival of the Sluttiest

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Survival of the Sluttiest index

What are babies made of? A sperm and an egg, of course. Or, to hark back to grade school sex education films, a fat and passive cartoon egg, poked at by a wriggling mass of desperate sperm — all from the same male, of course. Not so, writes behavioral ecologist Tim Birkhead in his unsettling new book Promiscuity: An Evolutionary History of Sperm Competition. According to Birkhead, most conceptions (including human ones) result from a spermatic gang-bang with one thrashing winner.

    

Okay, let’s just repeat that to make sure you understand the thesis: Birkhead states that most women have had sex with more than one man in the three to four days before they get pregnant. Even the most liberated readers of Nerve may be startled by this claim. Human females, Birkhead suggests, are far more naturally promiscuous than anyone ever thought, implying that a substantial fraction of offspring are not fathered by the mother’s regular partner.

    

All right then. Let’s proceed. We know, of course, that males tend to compete with each other for the favors of females. Either they fight, with the female as prize, or they woo, and the female chooses the one who either offers better resources or is more attractive. This kind of behavior is common throughout the animal kingdom, and it has a simple explanation: a male can inseminate an immense number of females, but a female needs very few inseminations to have all the offspring she can bear. Therefore, it pays for the female to be choosy and have sex with the “best” male, the one who is genetically favored or in a position to protect and support the offspring. By being promiscuous, Birkhead says, females ensure that the war between males, and the choice by females, actually continues within their own reproductive tract. The foot soldiers in this war are the sperm themselves.

    

The book includes some striking stories to document the truth of this claim within a variety of non-human species. One amazing sequence of time-lapse photographs shows two sperm (possibly from different males) that have already penetrated the egg of a “comb jelly” (a marine invertebrate). During the sequence, the nucleus of the egg moves toward each sperm in turn, as if checking it out, before fusing with one of them. Birkhead also tells of how an insect called Xylocoris (a relative of the bedbug) engages in a kind of homosexual rape to facilitate sperm competition: one male injects his sperm into another male, so that the victim, when he next has sex with a female, unwittingly inseminates her with a mixture of his own and his rapist’s sperm. Finally, Birkhead describes how among supposedly monogamous birds, females slip away from the nest for some hanky-panky with neighboring males — again, to promote sperm competition.

    

But Birkhead gets all tangled up when he tries to extend the discussion to humans. You might imagine that DNA testing would have revealed the prevalence of rampant female promiscuity, but according to Birkhead, there are no published studies using this technique on humans. And Birkhead never goes on to offer any meaningful evidence to support the radical claims he makes early in the book. Indeed, other molecular studies (such as blood-group analysis) have suggested that the vast majority of children are fathered by their mother’s husband or regular partner — contrary to the notion of widespread sperm competition.

    

When Birkhead turns to a discussion of “polyandry” (women having more than one husband) in non-Western cultures, he simply runs into a brick wall, logic-wise. Polyandry is rare, limited to cultures so short on resources that one man cannot afford to support a wife on his own. Therefore, Birkhead simply concludes that “the potential for sperm competition among humans is probably rather limited.” A lame conclusion indeed after such a jaw-dropping opening argument! As the book progresses, Birkhead consistently fails to draw sharp distinctions between radically disparate types of human sexual behavior, never holding them to the comparatively rigorous definition of “sperm competition” he applies to animals.

    

Birkhead’s book dramatically illustrates two of the limitations of evolutionary psychology. Simple-minded analogies between human behavior and animal behavior are risky at best, irresponsibly goofy at worst. And since, like many evo-psych enthusiasts, Birkhead bases his arguments on anecdotes or assertions, not quantitative evidence, he contradicts himself when it suits him. For example, at one point he says that “ejaculation carries little cost and there has therefore probably been little selection (evolutionary pressure) against males copulating with the wrong species.” At another point he says that “the cost of producing sperm is probably not trivial and selection would rapidly penalize those males who were profligate and favor those who use their sperm prudently.” The reality is that no one knows what the “cost” of producing sperm is, so Birkhead shouldn’t be using assertions about that cost to make arguments in either direction.

    

Men or women, gay or straight — we are all creatures of evolution. But to determine how this shapes our sexual choices? That will take a richer, more nuanced analysis than Birkhead provides here.